Combustible gas sensor

ABSTRACT

A matched catalytically inactive compensator for use in a combustible gas sensor is made from a catalytically active detector. The catalytically active detector is exposed to a gas phase catalytic inhibitor such as hexamethyldisiloxane to completely destroy its catalytic ability thereby forming a compensator which has very similar chemical and physical properties to the untreated detectors.

FIELD OF THE INVENTION

The present invention relates to a catalytically inactive compensatorused with a detector in a combustible gas sensor and to the process forproducing the catalytically inactive compensator.

BACKGROUND OF THE INVENTION

It is known to use electrically heatable filaments typically in ahelical coil configuration for the detection of combustible gases inair. These filaments are usually embedded within or completelysurrounded by an oxide or other refractory material such as alumina orsilica so as to form a bead or pellet. Typically a pair of such filamentpellets is arranged in a Wheatstone bridge circuit to form a combustiblegas sensor. These filament pellets constitute the detecting andcompensating pelements respectively.

The detecting pelement is usually formed by the addition of acatalytically active material such as palladium or platinum to therefractory material from which the pellet is made. Alternatively, thefilament itself can be made from the catalytically active material.Similarly, the compensating pelement can be usually made by adding asuitable metallic oxide such as chromium oxide or some other catalystpoison or catalytic inhibitor to the material from which the pellet isformed. Such a combustible gas sensor is described in U.S. Pat. No.3,092,799.

Many different methods have been proposed for making a catalyticallyinactive compensator for a combustible gas sensor. In U.S. Pat. No.4,355,056, the compensator can be formed by coating the pelement withmetals to form an inactive adherent oxide layer upon oxidation. In U.S.Pat. Nos. 4,123,225 and 4,072,467, the compensator consists of ahomogeneous mixture of a non-catalytic carrier material such as aluminaand a catalyst poison such as potassium hydroxide to suppress theoxidation of combustible gases. In U.S. Pat. Nos. 4,315,956 and4,332,772, the compensator is covered with an aqueous solution ofcobaltous nitrate and then heated to form a cobalt oxide coating in anattempt to render the compensator catalytically inactive. U.S. Pat. No.4,313,907 discloses a compensator which is formed by applying a liquidcoating of either sodium metasilicate or lead borate in an attempt torender it catalytically inactive. In U.S. Pat. No. 4,045,177 thecompensator is coated with an oxide selected from the group consistingof vanadium, columbium, tantalum, chromium, molybdenum, tungsten oruranium to increase the emissivity of the compensator.

One major disadvantage with these compensating elements is that they arephysically and chemically different from the detecting element withwhich they are paired. As a result, they cannot exactly compensate forthe changes in temperature, humidity and pressure in the operatingenvironment of the sensor when placed in the Wheatstone bridge circuit.It would be desirable, therefore, to have a catalytically inactivecompensator which was practically identical in composition and physicalstructure to its corresponding detector so that these inherentdifferences are minimized.

SUMMARY OF THE INVENTION

Generally, the present invention relates to a matched catalyticallyinactive compensator created from a detector which has been renderedcatalytically inactive by exposure to a gas phase catalytic inhibitorsuch as a silicone-based compound, a phosphorous-based compound orhydrogen sulfide. Preferably the detector comprises a wire coil madefrom a catalytically active material such as palladium or platinum. Thecoil is embedded within and surrounded by a refractory material to forma pelement. Alternatively, the catalytically active material can becontained within or coated on the refractory material of the pelement.The compensator is formed by exposing the detector to a gas phasecatalyst poison, preferably hexamethyldisiloxane ordecamethylcyclpentasiloxane, to render it catalytically inactive andthen heating the detector to permanently set the catalytic inactivityand to remove any residual catalyst poison.

The present invention also relates to the method of converting adetector into a compensator by exposing it to a gas phase catalyticinhibitor, preferably having a high vapor pressure, to render thedetector catalytically inactive and thus suitable for use as acompensator in a combustible gas sensor. This method of formingcompensators from actual detectors enables the detectors to more easilybe matched with the compensators since their chemical and physicalproperties are almost identical.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The compensator of the present invention comprises a detector pelementwhich has been catalytically deactivated by exposure to a gas phasecatalytic poison such as a silicone-based compound or aphosphorous-based compound such as dimethylmethylphosphonate.Preferably, the detector pelement comprises a wire coil made of acatalytically active material such as platinum or palladium, and aceramic bead made of a refractory material such as alumina or silicawhich serves as a supporting framework and completely surrounds the wirecoil. The ends of the wire coil project from the ceramic bead ofrefractory material and can be electrically connected with a compensatorpelement in a measuring circuit, typically a Wheatstone bridge circuit.The compensator of the present invention is formed by infusing adetector pelement with a silicone-based compound preferably of a highvapor pressure such as hexamethyldisiloxane to make the detectorincapable of catalyzing a combustible material and thereby forming acompensator. Any silicone-based compound the vapor of which can diffuseinto the pelement can serve as the catalytic poison.

The detector is an active pelement that catalytically burns combustiblegases present in the sensor. The compensator, on the other hand, doesnot react to the presence of combustible gases but does changeproportionally to the detector when the operating environment of thesensor changes with respect to temperature, humidity and pressure. Thecompensator is wired electrically in the Wheatstone bridge circuit sothat its changes are reflected opposite to those of the detector,thereby providing for the stabile operation of the sensor when changestake place in the operating environment.

The compensator of the present invention provides several advantages.Initially it should be noted that only one type of pelement needs to bemanufactured, namely detectors. Moreover, since the compensators aremade from the detectors, the physical and chemical similarity of thepelements is practically identical thereby simplifying the task ofmatching the detectors and compensators used in a combustible sensor.Other advantages are also expected from the compensator of the presentinvention such as a longer useful life and a more quiet, drift-freeoperation thereby permitting the use of higher gain electronics with thesensor.

Preferably the compensator of the present invention is created bystarting with a detector made from many known processes including theone as described above. Initially, the detector is heated until thepelement turns a dull red color, typically 600° C. This heating cleansthe detector pelement by removing water and other impurities. Next, thedetector is exposed to a gas phase catalytic inhibitor such as asilicone-based compound. Preferably this is done by placing the detectorabove a liquid bath of hexamethyldisiloxane for thirty minutes. Due tothe high vapor pressure of hexamethyldisiloxane, this can be done atroom temperature and ambient pressure. Finally, the pelement is heatedagain to a dull red to permanently fix the catalytic inactivity and toeliminate any residual catalytic inhibitor which could poison thedetector with which this compensator will be matched when it is placedin a sensor. The initial heating to clean the detector is not necessarybut it greatly improves the manufacturing yield.

While a presently preferred embodiment of practicing the invention hasbeen shown and described with particularity, the invention may otherwisebe embodied within the scope of the following claims.

What is claimed is:
 1. In a sensor for detecting a combustible gascomprising a catalytically active detector and a catalytically inactivecompensator, the improvement comprising making the sensor from aplurality of chemically and physically similar catalytically activeelements one of said elements being the catalytically active detectorand creating the catalytically inactive compensator by taking another ofthe chemically and physically similar catalytically active elements andrendering it catalytically inactive by exposure to a gas-phase catalyticpoison.
 2. The sensor as described in claim 1 wherein the gas-phasecatalytic poison used to transform one of the catalytically activeelements into the catalytically inactive compensator is a silicone-basedcompound.
 3. The sensor as described in claim 2 wherein thesilicone-based compound has a vapor pressure sufficient to enable thecompound to diffuse into one of the catalytically active elements andrender it catalytically inactive thereby forming the catalyticallyinactive compensator.
 4. The sensor as described in claim 3 wherein thesilicone-based compound is hexamethyldisiloxane ordecamethylcyclpentasiloxane.
 5. The sensor as described in claim 1wherein the gas-phase catalytic poison used to transform one of thecatalytically active elements into the catalytically inactivecompensator is hydrogen sulfide or a phosphorous-based compound.